Standard Natural Rubber

Figure 1. 50-Volume loading of nylon fibers in standard natural rubber formulation...

Compared with the natural material, raw SBR is more uniform in a variety of ways. Not only is it more uniform in quality so that compounds are more consistent in both processing and product properties but it is also more uniform in the sense that it usually contains fewer undesired contaminants. In addition, over a period of years it has been generally less subject to large price variations. These differences in uniformity have, however, tended to lessen with the advent of improved grades of natural rubber such as Standard Malaysian Rubber which have appeared in recent years. 

Another approach has been adopted by the Du Pont Company with Adiprene C. This is a urethane-type polymer with unsaturated groups in the polymer. Because of the unsaturation the polymer may be vulcanised with sulphur, the standard vulcanising agent of the rubber industry. This is a clear-cut example of a product being modified to suit the processor rather than that of a processor adapting himself to meet new products. Whereas Adiprene C has poor tensile strength when unfilled, the use of carbon black leads to appreciable reinforcement (as is the case with SBR and to some extent natural rubber. 

In solid form, the natural rubber is graded according to the content of dirt remaining from the precipitation of latex at the plantation. Eight basic NR types have been traditionally recognized internationally. Only the so-called ribbed smoked sheets and the pale crepes are normally used for adhesives. The predominant grade system, the Standard Malaysian Rubber system, has been used since 1965. 

Standard-grade PSAs are usually made from styrene-butadiene rubber (SBR), natural rubber, or blends thereof in solution. In addition to rubbers, polyacrylates, polymethylacrylates, polyfvinyl ethers), polychloroprene, and polyisobutenes are often components of the system ([198], pp. 25-39). These are often modified with phenolic resins, or resins based on rosin esters, coumarones, or hydrocarbons. Phenolic resins improve temperature resistance, solvent resistance, and cohesive strength of PSA ([196], pp. 276-278). Antioxidants and tackifiers are also essential components. Sometimes the tackifier will be a lower molecular weight component of the high polymer system. The phenolic resins may be standard resoles, alkyl phenolics, or terpene-phenolic systems ([198], pp. 25-39 and 80-81). Pressure-sensitive dispersions are normally comprised of special acrylic ester copolymers with resin modifiers. The high polymer base used determines adhesive and cohesive properties of the PSA. 

The standard polymers used for rubber linings consist of materials that are cross-linkable macromolecules which, on mixing with suitable reactants that form strong chemical bonds, change from a soft deformable substance into an elastic material. These polymers include natural rubber and its corresponding synthetic, c/s-polyisoprene, styrene-butadiene rubber, polychloroprene, butyl rubber, halogenated butyl rubbers, acrylonitrile-... 

Standard butyl rubber, which is a copolymer of isobutylene with about 2% of isoprene vulcanises in the same manner as natural rubber but, as it only contains a small proportion of polyisoprene, the cross-link percentage is much reduced. It is therefore not possible to make ebonite from a butyl rubber. The same vulcanisation chemistry, with some modifications, applies to ethylene-propylene terpolymers and brominated butyl rubber. 

In the past chemical cure linings have been employed on a wide scale. These linings, usually based on natural rubber or acrylonitrile-butadiene rubber consist of a standard lining compound with a chemical activator such as dibenzylamine incorporated in the formulation. Prior to the application of the lining to the substrate, the individual sheets of rubber are dipped or brush coated with carbon disulphide or a solution of a xanthogen disulphide in a solvent. The carbon disulphide or xanthogen disulphide permeates the rubber and combines with the dibenzylamine to form an ultra-fast dithiocar-bamate accelerator in situ, and thus the rubber rapidly vulcanises at ambient temperature. 

Whilst some standard elastomers such as natural rubber can be compounded to withstand many abrasive conditions, and to have an acceptable life span. 

A test which is claimed to be a more reliable guide to the condition of a natural rubber latex than the KOH number. The test is performed by steam distillation of the latex and titration of the distillate with standard alkali solution. The VFA number is a measure of the state of degradation of the latex. 

The remaining latex is processed into dry rubber as sheets, crepes and bales. There is an International Standard for the Quality and Packing for Natural Rubber grades, the so-called Green Book , published by the Rubber Manufacturers Association. The following grades of NR listed in the Green Book are sold to visual inspection standards only ... 

Also, the high mobility present in elastomers creats a weak dipolar coupling so that the cross polarization is inefficient and results in weak enhancement compared to standard free induction decay spectra. As far as material identification is concerned, the spectrum resulting from acquiring a standard pulsed free induction decay at an elevated temperature is adequate. Further research will probably show the narrow lines from the magic angle spectra of natural rubber may allow assignments to lesser components. ... 

Although a large number of synthetic elastomers are now available, natural rubber must still be regarded as the standard elastomer because of the excellently balanced combination of desirable qualities. The most important synthetic elastomer is styrene-butadiene rubber (SBR), which is used predominantly for tires when reinforced with carbon black. Nitrile rubber (NR) is a raudom copolymer of acrylonitrile and butadiene and is used when an elastomer is required that is resistant to swelling in organic solvents. 

The Green Book," International Standards of Quality and Packingfor Natural Rubber Grades, Rubber Manufacturers Association, Inc., Washington, D.C., 1975. 

ISO 1658 Natural Rubber - Test Recipes and Evaluation of Vulcanisation Characteristics, International Oiganization for Standardization, Geneva,... 

In 1941, exploratory work on incendiary materials was begun under L.F. Fieser at Harvard University (Ref 7). The first material developed was gasoline (80-octane motor-vehicle type), thickened (jellied) by addition of natural robber. This was a satisfactory product but, due to the shortage of natural rubber, it could not be produced on a large scale. Therefore, a new thickener was developed which consisted of erode aluminum naphthanate modified by addition of A1 soaps of cocoanuc oil acids. This thickened gasoline was not very successful at first (See further). Standard Oil Co (Esso)... 

Raw rubber can be either natural or synthetic. The standard grades of natural rubber are smoked sheets and pale crepe. For high grade products, first grade smoked sheets, namely RMAIX or Pale Crepe, is used. Pale Crepe is practically colourless and is preferred in products where delicate colour, extreme whiteness or transparency... 

Fig. 49. Standard 13C NMR spectrum (top) and DEPT spectra of sulfur cured natural rubber (6h at 138 °C). The label T indicates peaks from trans-polyisoprene units, X marks residual peaks from other subspectra and arrows indicate peaks due to crosslink sites (adapted from Ref. 194>)...

Nakajima and Hamel50 have derived expressions for calculating shear stresses from the Mooney torque values to give viscosities in agreement with those obtained from other instruments, and also an expression to correct for the edge effects.51 Bristow52 derived non-standard Mooney parameters for natural rubbers to improve the distinction between different grades. 

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